Ethylene.propylene.diene copolymer rubber (EPDM) has no double bond in the main chain of the molecular structure thereof and, therefore, exhibits excellent heat aging resistance, weather resistance, and ozone resistance as compared with general-purpose conjugated diene rubber, and has been applied widely to uses, e.g., automobile components, wire materials, electric electronic components, construction and civil engineering materials, and industrial materials and components.
The EPDM is used for, for example, automobile exterior materials e.g., glass run channels and weather strip sponges, and radiator hose materials. In recent years, the need for weight reduction of products has been intensified and the EPDM exhibiting excellent foamability has been desired.
Control of a vulcanization reaction and a foaming reaction is required for preparing a foam produced from a composition containing the EPDM. Furthermore, it is necessary to increase the viscosity of the foaming medium and the effective amount of the produced gas (gas-retaining property) in order to improve the foamability. At that time, it is believed that an increase in high-molecular weight component of the EPDM and introduction of a branch structure in the EPDM are effective for the purpose of improving the gas-retaining property and the shape stability (refer to Non-Patent Documents 1 and 2, for example). However, a composition containing the EPDM having an adequate foamability has not yet been proposed.
Moreover, a sponge rubber which has an excellent shape-retaining property while being lightweight and flexible has been desired as the material for the above-described uses, and a cross-linked foam has been proposed, which is produced from the EPDM containing at least one diene component or a composition thereof. For example, Patent Document 1 discloses an ethylene.α-olefin.nonconjugated diene copolymer rubber, in which 5-ethylidene-2-norbornene (ENB) is used as the diene component, which exhibits excellent shape-retaining property and kneadability, and in addition, which is excellent in compression set. Patent Document 2 discloses an ethylene.α-olefin.nonconjugated diene copolymer rubber, in which ENB and dicyclopentadiene are used as nonconjugated dienes, and discloses a sponge, in which the above-described rubber is used, which is highly foamed, and which is excellent in compression set. Regarding the compositions containing the EPDM described in Patent Documents 1 and 2, the extrudability and the balance between the vulcanization rate and the mechanical strength of the vulcanized rubber are improved as compared with those of the conventional composition containing the EPDM, but a composition containing the EPDM and having an adequate balance has not yet been produced. Furthermore, in order to produce the EPDM described in Non-Patent Documents 1 and 2, it is necessary to use large amounts of expensive nonconjugated polyene, e.g., ENB or VNB. Therefore, there still is room for improvement from the viewpoint of reduction in production cost.
Under such circumstances, it is desired that a composition capable of providing a cross-linked foam, which has a low specific gravity and an excellent shape-retaining property and which is excellent from the viewpoint of the production cost, is further proposed.